This invention relates to a color display device comprising a color display tube provided with a deflection unit, an electron source for generating three electron beams, a phosphor screen and an interpositioned shadow mask, said device further comprising means for correcting landing-errors.
Color display tubes have a phosphor screen with discrete areas each emitting light of a given color. For example, red light is emitted when a "red" electron beam impinges upon an area emitting red light (dot, strip or other geometrical shape). To prevent excitation of other areas by the "red" beam, a shadow mask provided with a pattern of apertures is used. It is positioned in such a way that the "red" electron beam passes the apertures at a given angle so as to impinge upon the red areas, while the other light-emitting areas are shielded from the red beam by the fixed parts of the mask. In a comparable manner, a "blue" electron gun produces a "blue" electron beam which passes the apertures at a slightly different angle so as to impinge only upon phosphors which emit blue light. The same applies to green light.
To produce light in areas which are remote from the centre of the face plate of the tube, the deflection unit is energized. The electron paths are thereby curved gradually, but the effect is as if they are bent at an acute angle at points which are known as color centres and are located on the lines of intersection of the undeflected paths and a hypothetical deflection plane; thus there is a red, a green and a blue color centre. It will be evident that a line drawn from a color centre through a shadow mask aperture must also pass through a phosphor dot which emits light whose color corresponds to that of the color centre in question. Displacement of either the color centre, the shadow mask (and its apertures), or the phosphor areas may cause an electron beam to impinge upon areas emitting a different color than the desired color, i.e. the beam lands incorrectly. The extent of displacement of an electron spot on the phosphor screen without a spot landing on an area having the wrong color is referred to as a guardband.
Landing-errors in shadow mask tubes are the result of manufacturing tolerances, influences of (earth's) magnetic fields, and doming. To prevent these errors from leading to color and luminance errors, a given extent of guardband is built into the design of the tube. However, this is detrimental to the total luminance.
Thus, by preventing or correcting the landing-errors, a considerable advantage can be obtained.
It is known that landing-errors in shadow mask tubes can be corrected by means of a current through an extra dipole (2-pole-x) coil system with which a transversal field is generated, "transversal" meaning transverse to the z-direction. A field in the y direction has, for example, an influence on the electron paths in the x direction and is therefore called "2-pole-x" field. A field in the x-direction is called a 2-pole-y field.
Upon energizing the 2-pole-x coil system, convergence and raster errors in both the x and the y direction are produced as side effects. The x raster error is the largest, but in practical cases, many of, in some cases all of these errors are found to be unacceptable if it is necessary to control a coil system for generating a useful extent of landing-correction (for example, 40 .mu.m in a 32 inch Wide Screen tube).